1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire for passenger car controlling the degradation of wet performances such as gripping performance, braking performance and the like on wet road surface at a time from middle worn stage to last worn stage.
2. Description of Related Art
In general, when the tire is mounted onto a vehicle and run over a long distance, it is gradually worn. In this case, tire performances in a new tire are not necessarily the same as tire performances at last worn stage. Particularly, the change of performances on wet road surface is large, which is considered due to the change of rubber properties in addition to a great factor that the volume of the grooves forming a tread pattern is decreased by wearing to lower the drainage efficiency against puddles existing on road. However, it can not be avoided to wear the tire to decrease the groove volume.
In order to control the degradation of wet performances even if the groove volume is decreased by the wearing of the tire, there is proposed a countermeasure wherein rubber having a high friction coefficient xcexc against wet road surface (hereinafter referred to as wet-xcexc) is exposed from a tread rubber at last worn stage to supplement the lowering of the performances due to the decrease of the groove volume with such a high wet-xcexc. When the tread rubber of the tire has a two-layer structure of a cap rubber layer and a base rubber layer, if rubber having a high wet-xcexc is simply applied to the base rubber layer, there is caused a problem that work load in the extrusion operation of base rubber is increased to lower production efficiency.
It is, therefore, an object of the invention to provide a pneumatic radial tire for passenger car wherein wet-xcexc is effectively enhanced at a time from middle worn stage to last worn stage to control the degradation of wet performances without obstructing the productivity.
The inventors have made various studies with respect to the compounding of base rubber from viewpoints of material, production efficiency and performances, concretely the structure and arrangement of the base rubber effectively increasing wet-xcexc without obstructing the productivity as well as the compounding of the base rubber controlling the change of rubber properties in use and found that the above problem can be solved by rendering the tread portion into a specified cap/base structure and compounding a recrosslinking inhibition assistant and an inorganic compound powder into the base rubber layer and as a result, the invention has been accomplished.
According to the invention, there is the provision of a pneumatic radial tire comprising a pair of ring-shaped bead cores, a bead filler disposed on the bead core, a carcass toroidally extending between the bead cores and comprised of a rubberized ply containing plural cords arranged side by side in a radial direction and wound around the bead core from inside toward outside to form a turnup portion, a belt of plural belt layers arranged outward on the carcass in the radial direction, an annular tread portion disposed outward on the belt in the radial direction, and a pair of sidewall portions extending from both ends of the tread portion; said tread portion having a two-layer structure of a cap rubber layer disposed outward in the radial direction and a base rubber layer disposed inward in the radial direction, and an outer surface of said base rubber layer in the radial direction being existent in a position corresponding to not less than 20% of a depth of a main groove formed in said tread portion outward from a bottom of said main groove in the radial direction, and said base rubber layer being compounded with a recrosslinking inhibition assistant and/or an inorganic compound powder.
In preferred embodiments of the invention, the base rubber layer is compounded with 0.5-5.0 parts by weight of the recrosslinking inhibition assistant based on 100 parts by weight of rubber ingredient; and the base rubber layer is compounded with not less than 10 parts by weight of the inorganic compound powder based on 100 parts by weight of rubber ingredient; and the inorganic compound powder is compounded into the base rubber layer in an amount of 10-100 parts by weight based on 100 parts by weight of rubber ingredient and into the cap rubber layer in an amount of not more than 50 parts by weight based on 100 parts by weight of rubber ingredient, respectively, provided that the compounding ratio of the inorganic compound powder in the base rubber layer is higher than that in the cap rubber layer; and the recrosslinking inhibition assistant is compounded into the car rubber layer; and the inorganic compound power is silica or aluminum hydroxide; and the recrosslinking inhibition assistant is a metal salt of dithiophosphoric acid, a thiuram type vulcanization accelerator or a 4-methylbenzothiazole type compound; and the outer surface of the base rubber layer in the radial direction is existent in a position from the bottom of the main groove within a range of 30-90%, preferably 40-60% of the main groove depth; and a volume ratio of cap rubber layer to base rubber layer is 1:0.5-1.0; and the base rubber layer is arranged on a central zone of the tread portion at a width corresponding to 40-80% of a ground contact width of the tread portion; and a volume ratio of cap rubber layer to base rubber layer in each of blocks forming the tread pattern at a distance from the bottom of the groove to the outer surface of the tread portion in the radial direction is 9:1-2:8; and the base rubber layer having a convex structure is a main part of a ground contact region at a middle worn stage and then contacts with ground as a main part until at least an appearance of slip sign; and a ratio of the base rubber layer contacting with ground gradually increases as compared with a ratio of the cap rubber layer contacting with ground with the advance of wearing from the middle worn stage; and the friction coefficient of the base rubber layer on wet road surface is at least 105% of that of the cap rubber layer; and the cap rubber layer has a thermal curing ratio of 100-160% after a modulus of elasticity at 300% elongation (M300) is held at 100xc2x0 C. for 48 hours and the base rubber layer has a thermal curing ratio of 100-150%, provided that the thermal curing ratio of the base rubber layer is lower than that of the cap rubber layer; and the base rubber layer contains not more than 55 parts by weight of an oil based on 100 parts by weight of rubber ingredient; and the base rubber layer contains short fibers.
The term xe2x80x9cmiddle worn stagexe2x80x9d used herein means a point of time that the wearing proceeds from the outer surface of the tread portion to a position corresponding to 40-60% of the depth of the main groove in the radial direction of the tire.